Method of fabricating photonic glass structures by extruding, sintering and drawing

ABSTRACT

Disclosed is a method of making a photonic crystal using a combination of extruding and drawing techniques. The method is contemplated as being capable of producing both two and three dimensional crystals due to the maturity and diversity of extruding and drawing technology. The method allows the drawing of relatively large photonic crystals and is flexible enough to provide a periodic array of channels or filaments as the crystal features. After the extruding step or steps and before the step of heating and drawing, a plurality of elongated extruded bodies can be bundled and drawn as a unit.

This application claims the benefit of U.S. provisional application No.60/094,609, filed Jul. 30, 1998.

BACKGROUND OF THE INVENTION

The invention relates to a method of making photonic crystals andpassive components comprising photonic crystals. In particular, themethod includes one or more extrusion steps to produce a cellular orchanneled object followed by a step of viscously sintering the object.The sintered, channeled object is heated and drawn to a final diameter.

A photonic crystal is a structure having a periodic variation indielectric constant. The periodic structure may be 1, 2 or 3dimensional. The photonic crystal allows passage of certain lightwavelengths and prevents passage of certain other light wavelengths.Thus the photonic crystals are said to have allowed light wavelengthbands and band gaps which define the wavelength bands which are excludedfrom the crystal.

At present, the wavelengths of interest for telecommunicationapplications are in the range of about 800 nm to 1800 nm. Of particularinterest is the wavelength band in the range of about 1300 nm to 1600nm.

Light having a wavelength in the band gap may not pass through thephotonic crystal. Light having a wavelength in bands above and below theband gap may propagate through the crystal. A photonic crystal exhibitsa set of band gaps which are analogous to the solutions of the Braggscattering equation. The band gaps are determined by the pattern andperiod of the variation in dielectric constant. Thus the periodic arrayof variation in dielectric constant acts as a Bragg scatterer of lightof certain wavelengths in analogy with the Bragg scattering of x-rayswavelengths by atoms in a lattice.

Introducing defects into the periodic variation of the photonic crystaldielectric constant can alter allowed or non-allowed light wavelengthswhich can propagate in the crystal. Light which cannot propagate in thephotonic crystal but can propagate in the defect region will be trappedin the defect region. Thus, a point defect within the crystal can serveas a localized “light cavity”. Analogously, a line defect in thephotonic crystal can act as a waveguide for a mode having a wavelengthin the band gap, the crystal lattice serving to confine the guided lightto the defect line in the crystal. A particular line defect in a threedimensional photonic crystal would act as a waveguide channel, for lightwavelengths in the band gap. A review of the structure and function ofphotonic crystals is found in, “Photonic Crystals: putting a new twiston light”, Nature, vol. 386, Mar. 13, 1997, pp. 143-149, Joannopoulos etal.

A first order band gap phenomenon is observed when the period of thevariation in dielectric constant is of the order of the light wavelengthwhich is to undergo Bragg scattering. Thus, for the wavelengths ofinterest, i.e., in the range of about 1300 nm to 1600 nm, as set forthabove, a first order band gap is achieved when the period of thevariation is about 500 nm. However, photonic crystal effects can occurin crystals having dielectric periodicity in the range of about 0.1 μmto 5 μm. A two or three dimensional photonic crystal having even thislarger special periodicity is difficult to fabricate.

In U.S. Pat. No. 5,774,779, Tuchinskiy, a method of makingmulti-channeled structures is described. Rods are bundled together andreduced in diameter by extrusion. The step of bundling and extrusion maybe repeated using rods which have already been extruded one or moretimes. However, no step of drawing is disclosed, so that channeldensity, expressed as number of channels per unit area, is not largeenough to produce a photonic crystal.

There is a need for a method of making photonic crystals of two or threedimensions which is repeatable, versatile, and potentially adaptable toa manufacturing environment, as compared to that of a laboratory.

SUMMARY OF THE INVENTION

The primary object of the invention is to combine extrusion technology,including the technology of powder extrusion, with glass drawingtechnology to address the problem of fabricating photonic crystals ofall types. The term drawing describes a process in which a viscous bodyof material is stretched along a pre-selected dimension. To stretch theviscous body without causing tears in the body, the viscosity of thebody and drawing tension applied to the body are properly adjusted. Theviscosity of the body may be controlled by controlling the temperatureof the body.

A first aspect of the invention is a method of making a photonic crystalhaving a band gap. A material comprising at least one glass powder and abinder is extruded through a die to form a body having a first and asecond face spaced apart from each other, each face having a pluralityof openings. The respective openings in each face are the ends ofchannels, which extend along the dimension between the two faces.

Suitable glass powders for making the crystal include Pyrex™ andsubstantially pure silica powder. The extruded body is then heated todrive off the binder at a first temperature and further heated to ahigher second temperature to viscously sinter the particulate of theglass powder to form a sintered, extruded glass body. This sinteredglass body is further heated and drawn, along the dimension between thetwo faces, to reduce the diameter of the channels extending between thetwo channels. The drawn body is referred to as a glass rod or glassfiber having a plurality of channels which extend along the long axis ofthe fiber or rod. The drawing temperature is typically higher than thesintering temperature, although for certain glass compositions anddrawing tensions the drawing temperature may be lower than the sinteringtemperature.

An optional series of steps may be used if, after extrusion, the body istoo large to be accommodated in a drawing furnace. That is, the crosssectional area, taken perpendicular to the dimension between the twofaces, of the body and thus the size of the plurality of channels may bereduced by:

filling the channels with a pliable material;

passing the body, in a direction along the channels, through one or aseries of reducing dies; and,

removing the pliable material.

This pliable material, which may be a micro-crystalline wax as set forthin Provisional Application No. 60/068230, serves to maintain thechannels as the body is passed through one or a series of reducing dies.A reducing die may take the form of a funnel with an entrance opening ofdimension commensurate with the cross sectional dimension of the bodyand an exit opening reduced in size by a factor of 2 or more relative tothe entrance opening. After the reducing step, the pliable material isremoved

In order for the channeled glass fiber to function as a photoniccrystal, the array of channel openings is distributed periodicallyacross the faces of the fiber. For the wavelengths of particularinterest at this time in telecommunications, the period of the array ofthe final drawn fiber or rod is in the range of about 0.4 μm to 5 μm.The novel method disclosed and described herein can produce arrayshaving periods less than 40 μm, preferably less than 5 μm and mostpreferably less than 1 μm.

Also, the dielectric constant of the channels must be different fromthat of the material forming the walls of the channels by a factor ofabout 3 to provide a useful band gap. For example the channels may befilled with air or evacuated to provide the requisite difference indielectric constant. As an alternative the channels could be filled withessentially any solid or fluid having the appropriate dielectricconstant as compared to that of the glass body.

The required dimensions of a photonic crystal depend upon the intendeduse thereof. Of particular importance is the crystal area which will beilluminated by a beam of light incident upon the crystal which willpropagate through the crystal or a defect in the crystal. The area ofthe beam may be characterized, for example, by the mode field diameterof the beam. For wavelengths that are at present of greatest interest inoptical telecommunications, i.e., those in the range of about 1300 nm to1600 nm, mode field diameters may be expected to be less than about 10μm. Thus a reasonable length of photonic crystal measured along thelength of the periodic features, is in the range of 3 μm to 12 μm, inthe case of side illumination of the crystal.

The area of a plane perpendicular to the length extent of the photoniccrystal periodic features can be selected to be in the range of about100 μm² to about 1.25 mm². Larger cross sections are possible using abundling technique described herein. However, bundling is not wellsuited to providing uniform periodicity among the elements, such asrods, which make up the bundle. Maintaining common periodicity among thebundled elements is more feasible in the case of rods that can be givenan orientation relative to each other which is maintained during heatingand drawing. For example square, rectangular, or hexagonal shaped rodscan be arranged in a close pack or other pre-selected pattern that willpersist through the drawing step.

Such a choice of area is large compared to the light wavelengthpropagated and allows for line defects in the form of waveguide pathsfor couplers and splitters. However, it should be understood that thecalculation of a band gap in a photonic crystal, or in a photoniccrystal having a defect, contains the underlying assumption of a crystalstructure essentially infinite in extent. What constitutes a crystalhaving effectively “infinite” dimensions is a question that must beanswered by experiment.

In practice, the length of a photonic crystal made using the methoddisclosed and described herein is limited on the low end only by thetechnology available to cut a slice from the drawn glass body. Thepotential upper limit of length is very large when compared to thelength required in optical circuits. The method may reasonably beexpected to yield photonic fiber crystals having lengths of the order oftens of centimeters or more.

The glass material to be extruded has a particle size preferably lessthan about 5 μm. A more preferred particle size is about 1 μm. This sizeprovides for good cohesion of the extruded material while allowing forthe extruded wall thickness of the channels to be no less than 10particle diameters, a practical upper limit for both direct particulateextrusion and the optional reduction particulate extrusion. However,larger particle size can be used in cases where a large part of the sizereduction is done after the step of viscous sintering, because theparticles lose their identities during the sintering step.

Extrusion dies are available which can introduce local or line defectsinto the elongated body during the extrusion step. Thus a cavityresonator, a waveguide, or a plurality of waveguides may be formed inthe extrusion step. It will be understood that the integrity of theextruded body must be maintained during the extrusion steps. Thus in thecase of void type defects which pass completely across the face of thephotonic crystal, an outermost annular layer, i.e., a cladding layer,must be maintained though the draw step. After the drawing process, alayer designed to preserve the extruded body integrity may be removed byknown mechanical or chemical means. If the layer is transparent tosignal light, it may remain in place after drawing.

As an alternative, local or line defects can be created in the extrudedbody prior to drawing removing parts of the wall structure using eithermechanical or chemical means. As an alternative, defects can be createdby inserting or back-filling channels. If a reduction die extrusion isused, the embedding can be done before or after that step.

A particularly useful photonic crystal component is one having twointersecting waveguide paths. The crystal periodicity is chosen suchthat light propagating along the line defect, i.e., waveguide in thecrystal is in the band gap. Thus, even at a right angle intersection oftwo waveguide paths the propagating light will make the right angle turnwith essentially no loss. The only possible loss is that due to backscattering through the light input port. Here again it should be notedthat the statement that the light traverses a bend with essentially noloss contains the tacit assumption of infinite crystal extent.

The method is also adaptable to the making of optical waveguide fiberswhich have a particular pre-selected channel pattern which extends alongthe long axis of the waveguide and terminates at the ends of thewaveguide. It will be understood that other channel patterns may befound to be useful. For example, channels along the long axis may beintermittent, randomly distributed instead of periodic, or extend overonly a few segments of the waveguide length. Also, channels whichintersect the long axis, having either a periodic or random pattern, maybe found to produce a particular propagation property which is useful inoptical waveguide communication systems. Methods for producing channelswhich intersect the long axis include a piercing step which would becarried out during or after drawing.

An exemplary configuration that is worthy of study is one in which thecenter portion of the waveguide fiber is a solid glass. The centerportion of the waveguide is surrounded by and in contact with achanneled structure which in effect forms the cladding of the waveguide.Such structures have been found to provide waveguide fibers whichpropagate a single mode over an unusually wide wavelength range. See,for example, Birks et al., “Endlessly Single Mode Photonic CrystalFibers”, Opt. Lett. 22 (13), 961, (1997). The performance of such awaveguide may be expected to change as the number of channels changes,the periodicity changes, or more than one channel size is used. In thislatter case, two or more sizes of channels may be used, each of thesizes conforming to a selected periodicity pattern. The making of diesfor extrusion of paste or plastic materials is a mature art. The diesrequired for the line defect, the cavity defect, or the porous claddingin any of its combinations of size and periodicity are known. The dieswill therefore not be discussed further here.

For a discussion of the band gap associated with such multiple channelsize photonic crystal structures, see, for example, Anderson et al.,“Larger Two Dimensional Photonic Band Gaps”, Phys. Rev. Letters, V. 77,No.14, p. 2949-2952, Sep. 30, 1996. In this reference, examples ofstructures which have a band gap and in which the number of differentchannel sizes is 2 are described.

Another potentially useful embodiment of the method is an opticalwaveguide fiber that includes a center channel which is surrounded by aperiodic array of channels of smaller dimension. The existence of a bandgap for such a configuration exists in theory, but has not been verifiedby experiment. As noted above, the die technology exists for making suchextruded bodies for later, size reduction, viscous sintering, anddrawing to form a waveguide fiber.

In a second aspect of the invention, the method may be used to produce aplurality of glass rods or tubes as described in the first aspect above.Prior to the step of heating and drawing the glass rods into smallerdiameter rods or fibers, two or more rods may be bundled and drawn as aunit. This unit may be drawn in a single step or in drawing andrebundling steps that are repeated until a target size is reached. Theresulting elongated object can be:

a “polycrystalline” object, i.e., a cluster of photonic crystals havingthe same periodicity but not oriented such that the periodicity ismaintained from one photonic crystal to the next; or,

a cluster of photonic crystals having more than one periodicity, i.e.,more than one set of pass bands and band gaps.

As stated above, depending upon the bundling process and the shape ofthe bundled rods, the bundling process can produce either type ofpolycrystalline body or a body comprising a cluster of photoniccrystals.

A further aspect of the invention are the photonic crystals which can bemade using the methods disclosed and described herein.

Yet another aspect of the invention is a method of making a photoniccrystal in which two or more types of glass powder, having differentdielectric constants and mixed with one or more appropriate binders, areco-extruded to form an elongated body having a periodic array offilaments of one glass/binder type that extends from one end of the bodyto the other, separated from each other by walls comprising the otherglass/ binder type. In the art the glass/binder forming the walls issometimes referred to as the matrix glass. An alternative method ofmaking the crystal body containing at least two glass types includesbackfilling or stuffing the channels formed in an initial extrusion.Following the initial extrusion step, this aspect of the invention makesuse of essentially the steps as set forth in the first aspect of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a picture of an extruded structure which has been viscouslysintered.

FIG. 2a is a schematic illustration of a viscously sintered extrudedbody in a drawing furnace.

FIG. 2b is an illustration of a cross section of an extruded or a drawnbody.

FIG. 2c is an illustration of a bundle of extruded bodies prior todrawing.

FIG. 3 is a chart showing the pass bands and band gaps of a photoniccrystal.

FIG. 4 is an illustration of a cross section taken through the long axisof the extruded or drawn photonic crystal in which the periodic featuresare shown.

FIG. 5 is an illustration of a 2-D photonic crystal having a waveguidepath therethrough.

FIG. 6 is an illustration of a waveguide fiber having a solid glass coreand a photonic crystal cladding.

FIG. 7 is an illustration of a waveguide fiber having a large centralchannel surrounded by a photonic crystal comprised of smaller openingsor filaments.

FIG. 8 is an example of channels of different size overlapped to providea first periodic structure having a first channels size embedded in asecond periodic structure having a second channel size.

FIG. 9 is a drawing of a reducing die.

DETAILED DESCRIPTION OF THE INVENTION

The photonic crystal art is still in a rapidly developing stage in whichthe fundamental shapes, properties and uses of two or three dimensionalcrystals are being tested.

The extrusion process is distinctly suited for making a wide variety ofphotonic crystals because the extrusion art:

is mature in that processes are available for extruding a wide range ofmaterials and shapes;

is capable of producing periodic structures having very small wallthickness, a high degree of open frontal area, and a high channel (cell)density as measured in terms of number of channels (cells) per unitcross sectional area;

can be kept free of contamination, for example, by means of coated diesand mixers, so that the purity of extruded structures is limited only bythe purity of the starting materials;

is compatible with glass starting materials such as amorphous silicapowder mixed with a binder; and,

can provide, by means of reduction extrusion, a wide range of channeldensities simply by changing the cross section area of the body producedin the initial or reduction extruding step.

The extrusion process is documented elsewhere and so will not be setforth in detail herein. The extrusion methods and apparatus used in theinvention described herein may be found for example in U.S. Pat. No.3,790,654, Bagley and U.S. Pat. No. 4,902,216, Cunningham et al. thespecifications of which are incorporated into this document byreference. This technology is capable of extruding tens of thousands ofchannels simultaneously.

In an exemplary initial extrusion of a material, such as silica powderor Pyrex™ powder, through a die, a channel density of about 62 channelsper cm² has been demonstrated. Subsequent passing of the initialextruded body through a reduction die, which may be a funnel, canproduce a channel density of about 6200 channels per cm². Higher channeldensities are then achieved by hot drawing the viscously sinteredextruded substrate.

Referring to FIG. 9, reduction die body 78 is shown having an inputfunnel 74 into which the extruded body 72 is introduced. A mechanism forapplying forward force to the extruded body, not shown, causes theextruded body to pass through reducing section 76 and emerge as reducedbody 80.

After reduction extrusion, the minimum spacing between channel centers,assuming uniform wall thickness, is then of the order of 30 μm, a limitwhich is set by the desire that the minimum web thickness be about 10particle diameters. It is understood that this wall thickness can alsobe achieved by extruding followed by heating and drawing. That is, thereduction step is optional and typically is used in those cases wherethe drawing furnace dimensions are a limiting factor.

To reach a feature spacing for certain applications of a photoniccrystal, in the near infrared part of the spectrum, a spacing of aboutλ/3 or 0.5 μm is desired. However, the feature spacing may be largerthan the sub-micron level, of the order of several microns, and stillproduce useful photonic crystal structures. In either case the extrusionprocess can be used to fabricate the photonic crystal structure. Afterthe structure is extruded and then, optionally, further reduced bypassing the body through a reduction die, the resulting body is heatedto drive off the binder and then viscously sintered to produce a bodywhich is heated and drawn to further reduce the thickness dimension ofthe body and reach the desired pitch, of the order of tenths of microns,characteristic of an optically active structure.

The combination of extrusion technology, with that of drawing, providesa unique and versatile process for fabricating photonic crystals. Theextrusion process provides a wide range of cross sectional shapes whilemaintaining dimensional accuracy of the shapes. The dimensional accuracyof the drawing process, which meets or exceeds that of extrusion, makethe combination of these two processes a powerful tool in themanufacture of photonic crystals.

The periodic array need not include channels of any kind, because theextrusion process, e.g., a back fill or co-extrusion process, can makeuse of one or more starting materials and embed a periodic array of onematerial within the matrix of the other. That is, the photonic crystalis a solid glass body which exhibits a periodic variation in dielectricconstant of solid materials.

The viscously sintered or extruded body shown in FIG. 1 is a periodicarray of square channels. The body was made from an extrudate of silicapowder in a binder. The extruded body was heated to drive off thebinder. Subsequently the temperature of the body was increased toprovide for viscous sintering. The regularity of the array isnoteworthy. Depending upon the final configuration chosen for thephotonic crystal, either the channels 8 or the walls (web) 6 of theextruded and drawn body could function as the periodic feature of thecrystal.

A schematic representation of the drawing process is shown in FIG. 2a.The extruded, reduced, if needed, and viscously sintered body 10 issuspended in furnace 12. The suspending means, which include means forfeeding the glass body into the furnace, are not shown but are known inthe art. Coil 14 represents the heating element of furnace 12. Thelocalization of the hot zone near one end of the furnace provides afurnace temperature profile that allows a continuous and uniform glassrod or fiber 16 to be drawn from the sintered body 10 by gripping means18. Alternative gripping means are known, including types whichtranslate with the drawn rod or fiber 16. The extruded and reduced body10 may be viscously sintered during the drawing step if the tensilestrength of the unsintered body is sufficient to support the drawtension. As has been noted above, if the extruded body or extruded andsintered body has a small enough diameter, the reduction may beexcluded, and drawing begun immediately after the extrusion step. It isbelieved that the necessary geometry may be extruded into a preformhaving a diameter not substantially larger than about 5 cm, which is asize compatible with most draw furnaces.

In an alternative embodiment of the invention, the draw down process iscarried out in two steps. The viscously sintered body is first drawninto a rod or fiber which is then overclad and drawn to a fiber, whichtypically has a clad diameter of the order of 100's of microns and aphotonic crystal core diameter of 10's of microns. This process isexpected to be useful in obtaining structures having very high channeldensity. The cross section shown in FIG. 2b represents an extrudedchanneled body, an extruded and reduced channeled body, or an extruded,reduced and drawn channeled body. The channels are shown as periodicarray 22 which are formed in the glass 20.

It will be understood that the structure of FIG. 2b may be formed suchthat the periodic array 22 is an array of glass filament ends, in whichthe dielectric constant of glass array 22 is different from that ofglass matrix 20. Thus in describing a body having channels, applicantsat the same time are describing a body comprising a first and a secondglass one embedded in the other to form a periodic array of dielectricconstant differences.

An important feature of the invention is the capability of substantiallypreserving the extruded shape through subsequent process steps in whicha cross section of the shape is reduced in size.

To produce a photonic crystal having a pre-selected periodicity,illustrated as surfaces 28 in FIG. 2c, and a larger surface area, aplurality of extruded or extruded and reduced channeled bodies, 26 inFIG. 2c, may be bundled, using for example a surrounding tube 24 as adrawing aid, and drawn as a unit. As an alternative, channeled bodieshaving different periodicity may be bundled as shown in FIG. 2c anddrawn as a unit, thereby producing a set of photonic crystals havingdifferent band pass and band gap wavelengths. The units so drawn may beseveral interleaved periodicities and have different crystal structuresor be comprised of misaligned crystals. Some of the channeled bodies mayhave a periodicity intentionally made to be random. The need for data inthis field fits well with the versatility of the process combinationsdescribed and disclosed herein.

FIG. 3 illustrates the relative sizes of the pass band and band gapwavelength ranges. The frequency, in relative units, of the light isplotted on the y-axis versus the light wave vector on the x-axis, inrelative units. The first, second, and third band gaps, 30, 32 and 34,respectively, are shown as frequencies within the dashed lines. Theallowed or propagated frequency bands are above and below each of thegaps.

An embodiment of a photonic crystal made using the method describedherein is shown in FIG. 4. The extrusion steps and subsequent sinteringand drawing of the extruded body produces an elongated rod or fiber 40having a periodic array of channels 38 extending through matrix glass36.

Another embodiment of a photonic crystal made using the methoddisclosed, is illustrated in FIG. 5. Here the periodic pattern ofchannels has been altered to include two intersecting line defects 44and 46. The width of the line defect is chosen such that theintersecting line defects serve as waveguide paths for light having awavelength in the band gap of the crystal. The photonic crystal guidesthe light, even around a sharp bend, without producing any excess loss.Arrows 48 and 50 indicate a direction of travel of the light. Thewaveguide path is formed in the crystal in the first extruding step andmaintained therein through the heating and drawing step. Note that thedirection of light travel in this embodiment is at a non-zero angle withrespect to the photonic crystal centerline, so that the light enters andexits from a side of the photonic crystal rod or fiber.

A waveguide having a photonic crystal clad layer is illustrated in FIG.6. A solid glass core 56 is surrounded by photonic crystal 52. Thecircles 54 indicate the positions of material or the channels which formthe periodic array of dielectric constants. This waveguide structure,which is readily made by the novel method described herein, has beenfound (see reference above) to provide an unusually wide range ofwavelengths over which the waveguide propagates a single mode.

A contemplated waveguide structure which is thought to be of interest isillustrated in FIG. 7. In this case, the core region 58 is a hollowcylinder in the photonic crystal matrix glass 62. As before the circles60 represent the periodic dielectric constant array embedded in glass62. Circles 60 represent either channels or glass filaments extendingalong the waveguide length.

As an example of the versatility of the novel process, FIG. 8 shows twosets of periodic features 66 and 64 embedded in matrix glass 70. Thesets of circles and dots are each representative of a periodic array.The size or periodicity of the feature can be selected to provide aparticular band gap. Here again the extrusion technology is well suitedto making such overlapping or interwoven structures.

Although various embodiments of the invention have hereinabove beendisclosed and described, the invention is nonetheless limited only bythe following claims.

We claim:
 1. A method of making a photonic crystal which propagates apre-selected band of wavelengths and has a band gap comprising thesteps: a) extruding through a die a material comprising at least oneglass powder and a binder to form a body having a first face spacedapart from a second face, each face having an area, wherein a pluralityof channels extend from the first to the second face and form openingsin the respective faces, the channels separated one from another byintervening walls which have a cross section, the cross section of thewalls serving to separate the array of openings, one from another, inthe respective faces; b) heating the body to drive off the binder andviscously sinter the glass powder to form a glass body; c) drawing aglass fiber or rod from the glass body.
 2. The method of claim 1 whereinthe extruding step a) produces a periodic array of channels in the bodyand channel openings in the respective faces.
 3. The method of claim 2wherein the period of the array of openings is in the range of 0.4 μm to5.0 μm.
 4. The method of claim 3 further including the step, evacuatingthe channels or filling the channels in the glass fiber or rod with afluid, wherein evacuated channels have a first dielectric constant,fluid filled channels have a second dielectric constant, and the wallshave a third dielectric constant, to provide a periodic change indielectric constant over the first and second face and over any surfacearea of the glass fiber or rod which intersects the glass fiber or rodand is located between the first and second face.
 5. The method of claim4 in which the dielectric constant of the viscously sintered glasspowder is at least a factor of 3 greater than the dielectric constant ofthe evacuated or fluid filled channels.
 6. The method of claim 1 furtherincluding the step, prior to the drawing step c), heating the glass bodyto reduce the viscosity thereof.
 7. The method of claim 1 furtherincluding the steps after step c) of: overcladding the rod or fiber witha glass forming material; viscously sintering the glass forming materialto form an overclad rod or fiber; and, reducing the diameter of theoverclad rod or fiber by drawing.
 8. The method of claim 1 furtherincluding the steps, after step a): filling the plurality of channelswith a pliable material; extruding the body, in a direction parallel toan axis between the first and second faces, through at least onereducing die to reduce the areas of the first and second faces and thearea of any cross section of the body which is between the faces; and,removing the pliable material from the plurality of channels.
 9. Themethod of claim 8 in which the pliable material comprises amicro-crystalline wax.
 10. The method of claim 1 in which the area ofthe end faces after step c) is in the range of about 100 μm² to 1.25mm².
 11. The method of claim 1 in which the average particle size of theglass powder is about 5 μm.
 12. The method of claim 1 wherein said dieprovides for at least one line defect in the periodic array of channels.13. The method of claim 1 wherein said die provides for at least onecavity defect in the periodic array of channels.
 14. The method of claim1 wherein said die provides for a volume of material, which is free ofopenings, beginning at a centrally located area of the first face andextending along the body to a corresponding area of the second face. 15.The method of claim 1 wherein said die provides for a channel, beginningat a centrally located area of the first face and extending along thebody to a centrally located area of the second face, which comprises across section larger than the cross section of the channels surroundingthe centrally located channel, the larger channel being separated fromsurrounding channels by intervening walls.
 16. The method of claim 1wherein said die provides a plurality of channels having a crosssectional area of one of N sizes, where N is an integer, so that theplurality of channels forms N groups of channels, in which each channelof a group has a cross section of the same size, and in which each ofthe N groups of channels is periodically arrayed across each end face inone of N periodic arrays.
 17. The method of claim 16 in which N=2 andthe period of each of the two periodic arrays is different.
 18. A methodof making a photonic crystal which propagates a pre-selected band ofwavelengths and has a band gap comprising the steps: a) extrudingthrough a die a material comprising at least one glass powder and abinder to form a body having an axis between a first face spaced apartfrom a second face, each face having an area, wherein a plurality ofchannels extend from the first to the second face and form openings inthe respective faces, the channels separated one from another byintervening walls which have a cross section, the cross section of thewalls serving to separate the array of openings, one from another, inthe respective faces; b) heating the body to drive off the binder andviscously sinter the glass powder to form a glass body; c) drawing aglass fiber or rod from the glass body; d) repeating steps a) through c)to form a plurality of glass fibers or rods; e) bundling together atleast two of the glass fibers or rods using means for holding the atleast two glass fibers or rods in side by side registration along theaxes of the glass fibers or rods; f) drawing a glass fiber or rod fromthe bundled glass fibers or rods.
 19. The method of claim 18 wherein theextruding step a) produces a periodic array of channels in the body andchannel openings in the respective faces.
 20. The method of claim 19wherein the period of the array of openings is in the range of 0.4 μm to5.0 μm.
 21. The method of claim 20 further including the step,evacuating the channels or filling the channels in the glass fiber orrod with a fluid, wherein evacuated channels have a first dielectricconstant, fluid filled channels have a second dielectric constant, andthe walls have a third dielectric constant, to provide a periodic changein dielectric constant over the first and second face and over anysurface area of the glass fiber or rod which intersects the glass fiberor rod and is located between the first and second end face areas. 22.The method of claim 21 in which the dielectric constant of the viscouslysintered glass powder is at least a factor of 3 greater than thedielectric constant of the evacuated or fluid filled channels.
 23. Themethod of claim 18 further including the step, prior to the drawing stepc), heating the glass body to reduce the viscosity thereof.
 24. Themethod of claim 18 further including the steps after step c) of:overcladding the rod or fiber with a glass forming material; viscouslysintering the material to form an overclad rod or fiber; and, reducingthe diameter of the overclad rod or fiber by drawing.
 25. The method ofclaim 18 further including the steps, after step a): filling theplurality of channels with a pliable material; extruding the body, in adirection parallel to the axis between the first and second faces,through at least one reducing die to reduce the areas of the first andsecond faces and the area of any cross section of the body which isbetween the faces; and, removing the pliable material from the pluralityof channels.
 26. The method of claim 25 in which the pliable materialcomprises a micro-crystalline wax.
 27. The method of claim 18 furtherincluding the step, prior to the drawing step c), heating the glass bodyto reduce the viscosity thereof.
 28. The method of claim 18 furtherincluding the steps after step c) of: overcladding the rod or fiber witha glass forming material; viscously sintering the material to form anoverclad rod or fiber; and, reducing the diameter of the overclad rod orfiber by drawing.
 29. A method of making a photonic crystal whichpropagates a pre-selected band of wavelengths and has a band gapcomprising the steps: a) extruding through a die a material comprising afirst glass powder and a binder and a second glass powder and a binderto form a body having a first face spaced apart from a second face, eachface having an area, wherein a plurality of continuous filamentscomprising the first glass powder and binder extends from the first tothe second face, the filaments separated one from another by wallscomprising the second glass powder and binder, wherein the area of eachface comprises the filament ends comprising the first glass powder andbinder separated one from another by intervening wall cross sectionscomprising the second glass powder and binder; b) heating the body todrive off the binders and viscously sinter the first and second glasspowders to form a glass body; c) drawing a glass fiber or rod from theglass body.
 30. The method of claim 29 wherein the extruding step a)produces a periodic array of filaments in the body and filament ends inthe respective faces.
 31. The method of claim 30 wherein the period ofthe array of filaments is in the range of 0.4 μm to 5.0 μm.
 32. Themethod of claim 31 in which the filaments and filament ends have a firstdielectric constant, and the walls and wall ends have a seconddielectric constant, to provide a periodic change in dielectric constantover the first and second face and over any surface area of the glassfiber or rod which intersects the glass fiber or rod and is locatedbetween the first and second face.
 33. The method of claim 32 which thedielectric constant of the viscously sintered first glass powder is atleast a factor of 3 greater than the dielectric constant of theviscously sintered second glass powder.
 34. The method of claim 29 inwhich the average particle size of the first and second glass powder isabout 5 μm.
 35. The method of claim 29 further including the step, afterstep a): extruding the body, in a direction parallel to an axis betweenthe first and second face, through at least one reducing die to reducethe areas of the first and second faces and the area of any crosssection of the body which is between the faces.